Cirrus Logic AN311 - Manual (Page 4)

4 AN311REV1
AN311
With the ADC is configured to output words at 50 Sps, one can determine the attenuation of this filter at 60 Hz as
shown in the plot in Figure 3. From the plot, one can see that the Sinc
3 filter provides about -48 dB of attenuation
at 60 Hz (an actual calculation yields 48.4 dB). 48 dB is equivalent to a factor of 251 to 1. If the range from 57 Hz
to 63 Hz is examined (60 Hz, ±5%), the attenuation would vary from about -55 dB at 57 Hz to about -44 dB at 63 Hz.
At 63 Hz the 44 dB of attenuation would translate to a factor of 158 to 1. Therefore, with the converter set to output
words at 50 Sps, the filter attenuation at 63 Hz (60 Hz, +5%) would reduce any 63 Hz input component by the factor
of 158.
Another design option that can achieve much higher rejection while attenuating 50 Hz and 60 Hz simultaneously is
to operate the ADC from a 3.2786 MHz clock source.
Table 2 indicates the word rates available using either the 4.9152 MHz crystal or a 3.2768 MHz crystal. If the
3.768 MHz crystal is used, the FRS bit should be set to logic 0. Word rates of 5 and 10 Sps are then available.
When the output of the converter is set for 5 or 10 Sps (FRS = 0), the Sinc
3
filter provides simultaneous rejection of
50 Hz and 60 Hz line interference frequencies.
Figure 4. Third-order Sinc Filter Response (Sinc 3 )
With the output word rate at 5 or 10 Sps, the filter will produce three zeroes at each frequency that is an integer
multiple of the word rate. For example, at 10 Sps the filter will produce a triple zero at 10, 20, 30, 40, 50, 60, etc. as
shown in Figure 4.
Therefore, setting the converter to output either 5 or 10 Sps when operated from a 3.2768 MHz clock will provide
filter notches at both 50 Hz and 60 Hz and their harmonics.
Table 2. Crystal Frequency vs. Output Word Rate
Crystal Freq. FRS Available Word Rates
4.9152 MHz
0 3840, 1920, 960, 480, 240, 120, 60, 30, 15, 7.5
1 3200, 1600, 800, 400, 200, 100, 50, 25, 12.5, 6.25
3.2768 MHz
0 2560, 1280, 640, 320, 160, 80, 40, 20, 10, 5
1 2133.3, 1066.6, 533.3, 266.6, 133.3, 66.6, 33.3, 16.6, 8.3, 4.16
-120
-100
-80
-60
-40
-20
0
1 100 50
60
10
-18 dB per Octave
Frequency (Hz)
-60 dB per Decade